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参数资料
| 型号： | LTC3787IGN#TRPBF |
| 厂商： | LINEAR TECHNOLOGY CORP |
| 元件分类： | 稳压器 |
| 英文描述： | SWITCHING CONTROLLER, 585 kHz SWITCHING FREQ-MAX, PDSO28 |
| 封装： | 0.150 INCH, LEAD FREE, PLASTIC, SSOP-28 |
| 文件页数： | 14/36页 |
| 文件大小： | 363K |
| 代理商： | LTC3787IGN#TRPBF |
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LTC3787
21
3787fb
APPLICATIONS INFORMATION
Setting Output Voltage
The LTC3787 output voltage is set by an external feedback
resistor divider carefully placed across the output, as shown
in Figure 5. The regulated output voltage is determined by:
V
OUT = 1.2V
1
+
R
B
R
A
Great care should be taken to route the VFB line away
from noise sources, such as the inductor or the SW line.
Also keep the VFB node as small as possible to avoid
noise pickup.
INTVCC Regulators
The LTC3787 features two separate internal P-channel
low dropout linear regulators (LDO) that supply power at
the INTVCC pin from either the VBIAS supply pin or the
EXTVCC pin depending on the connection of the EXTVCC
pin. INTVCC powers the gate drivers and much of the
LTC3787’s internal circuitry. The VBIAS LDO and the
EXTVCC LDO regulate INTVCC to 5.4V. Each of these can
supply at least 50mA and must be bypassed to ground with
a minimum of 4.7μF ceramic capacitor. Good bypassing
is needed to supply the high transient currents required
by the MOSFET gate drivers and to prevent interaction
between the channels.
High input voltage applications in which large MOSFETs
are being driven at high frequencies may cause the maxi-
mum junction temperature rating for the LTC3787 to be
exceeded. The INTVCC current, which is dominated by the
gate charge current, may be supplied by either the VBIAS
LDO or the EXTVCC LDO. When the voltage on the EXTVCC
pin is less than 4.8V, the VBIAS LDO is enabled. In this
case, power dissipation for the IC is highest and is equal
to VBIAS IINTVCC. The gate charge current is dependent
on operating frequency, as discussed in the Efficiency
Considerations section. The junction temperature can be
estimated by using the equations given in Note 3 of the
Electrical Characteristics. For example, at 70°C ambient
temperature, the LTC3787 INTVCC current is limited to less
than 32mA in the QFN package from a 40V VBIAS supply
when not using the EXTVCC supply:
TJ = 70°C + (32mA)(40V)(43°C/W) = 125°C
In an SSOP package, the INTVCC current is limited to
less than 15mA from a 40V supply when not using the
EXTVCC supply:
TJ = 70°C + (15mA)(40V)(90°C/W) = 125°C
To prevent the maximum junction temperature from being
exceeded, the input supply current must be checked while
operating in continuous conduction mode (PLLIN/MODE
= INTVCC) at maximum VIN.
When the voltage applied to EXTVCC rises above 4.8V, the
VIN LDO is turned off and the EXTVCC LDO is enabled. The
EXTVCC LDO remains on as long as the voltage applied to
Figure 6. Using the SS Pin to Program Soft-Start
LTC3787
SS
CSS
SGND
3787 F06
Soft-Start (SS Pin)
The start-up of VOUT is controlled by the voltage on the
SS pin. When the voltage on the SS pin is less than the
internal 1.2V reference, the LTC3787 regulates the VFB
pin voltage to the voltage on the SS pin instead of 1.2V.
Soft-start is enabled by simply connecting a capacitor from
the SS pin to ground, as shown in Figure 6. An internal
10μA current source charges the capacitor, providing a
linear ramping voltage at the SS pin. The LTC3787 will
regulate the VFB pin (and hence, VOUT) according to the
voltage on the SS pin, allowing VOUT to rise smoothly
from VIN to its final regulated value. The total soft-start
time will be approximately:
t
SS = CSS
1.2V
10A
Figure 5. Setting Output Voltage
LTC3787
VFB
VOUT
RB
RA
3787 F05
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相关代理商/技术参数 |
参数描述 |
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| LTC3787IUFD#TRPBF | 功能描述:IC REG CTRLR BST PWM CM 28-QFN RoHS:是 类别:集成电路 (IC) >> PMIC - 稳压器 - DC DC 切换控制器 系列:PolyPhase® 标准包装:4,500 系列:PowerWise® PWM 型:控制器 输出数:1 频率 - 最大:1MHz 占空比:95% 电源电压:2.8 V ~ 5.5 V 降压:是 升压:无 回扫:无 反相:无 倍增器:无 除法器:无 Cuk:无 隔离:无 工作温度:-40°C ~ 125°C 封装/外壳:6-WDFN 裸露焊盘 包装:带卷 (TR) 配用:LM1771EVAL-ND - BOARD EVALUATION LM1771 其它名称:LM1771SSDX |
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